The present invention relates to radial piston pumps and more particularly, to radial piston pumps of the type used in fuel supply systems for internal combustion engines.
Radial piston pumps, particularly the type used for pressurizing fuel for delivery to the combustion chambers of internal combustion engines, typically have a housing defining a central cavity and a drive member mounted about a drive axis for rotation in the cavity. At least one piston bore extends radially relative to the axis, through the housing to the cavity. A piston oriented radially within the piston bore has a radially outer pumping end and a radially inner driven end cooperating with the drive member for reciprocal movement in the piston bore between top dead center and bottom dead center travel limits. A sliding shoe engages the driven end of the piston and bears on the drive member, for providing the cooperation whereby the rotary movement of the drive member is converted to the reciprocal movement of the piston. A return spring urges the driven end of the piston toward the shoe and the drive member. For the type of pump to which the present invention is especially directed, the drive member is eccentric, i.e., it has an outer circular surface with a center that is offset with respect to the drive axis. The driven end of the piston bears pivotally against, without being rigidly attached to, the shoe, to accommodate the eccentric path of the drive member.
Particularly in operational modes where the pumping chamber of the piston is not fully charged before the pressurization, or discharge, stroke of the piston, unbalanced forces can act on the shoe with potentially detrimental, if not disastrous, results. Such unbalanced forces can result in separation of the shoe from both the driven end of the piston and the drive member while the piston is at or near the top dead center position such that, despite the restorative forces of the return spring, the piston does not seat properly in the shoe, or in the worst scenario, the shoe is carried into the cavity, resulting in catastrophic damage to the pump.
One type of pump and associated control scheme in which this problem can arise, and for which the invention is particularly suited, is described in U.S. application Ser. No. 10/187,823, filed on Jul. 2, 2002, entitled “Hybrid Control Method in Fuel Pump Using Intermittent Recirculation at Low and High Engine Speeds”.